i BACKGROUND j TECHNICAL FIELD Embodiments of the subject matter disclosed herein relate to hybrid generator-battery systems and methods. Other, embodiments of the subject matter disclosed herein relate to control methods providing fuel savings for hybrid generator- battery systems. I DISCUSSION OF ART Battery applications are typically divided into two categories, backup and hybrid. The backup category relates to applications in which the battery is used as a i backup power source in case of main power source failure. The hybrid category ; relates to applications in which the battery undergoes continual (or in some cases, periodic) charge and discharge operation in concert with a main power source. ? Telecom operators in areas where grid power is unavailable or only I intermittently available have relied on diesel generators to power base transceiver ] stations (BTS). While inexpensive to install, the escalating cost of diesel fuel, and its delivery to remote locations, has driven the search for alternative solutions with lower total cost of ownership. Fuel usage can be dramatically reduced by use of a dieselfaM| battery hybrid power system. In such a scenario, a long life cycle battery is used to alternately share the load with the diesel generator. The diesel generator is modulated on and off and, when it is active, powers the BTS and recharges the battery at an overall higher efficiency than if powering the BTS only. Once the battery is recharged, the generator can be turned off and the battery is used to sustain the BTS load. Fuel savings of up to 50% have been achieved in some applications. Such hybrid systems may be used in other stationary power applications as well such as, for example, mining operations. Reduced fuel consumption directly impacts the operational expenditures of telecom sites and cuts greenhouse gas emissions. Hybrid systems may also be applied in mobile applications such as automobiles where an 2 t onboard generator is cycled on and off to maintain the charge or energy state of a battery. Other stationary or mobile applications are possible as well. For typical engine and generator sets used in telecom base stations, as the load fraction increases so does the efficiency of the engine and generator set. Load fraction = battery recharge power plus the base load divided by the rating of the generator source. Therefore, fuel savings is proportional to base load times battery discharge event time energy discharged divided by the sum of battery discharge event time plus battery recharge event time. Even with the success of the hybrid generator-battery systems in reducing i fuel costs, it is still desirable to further improve fuel savings over the potential life of { ^ P the equipment in such hybrid generator-battery systems. I BRIEF DESCRIPTION | Embodiments of the present invention address the applications of a i battery for hybrid installations. The value proposition in using a battery for the hybrid types of installation is maximized when the energy discharged from the battery can be | recharged in the shortest time period at a relatively high rate. As the recharge period shortens and the energy delivered from the battery to the load increases per day, the subsequent load fraction of the energy source (e.g., an engine and generator set) ! increases. | In one embodiment, a method is provided. The method includes controlling at least one of an applied recharge potential and/or a charge state window mk for an energy storage device (e.g., a battery power source) in response to at least one of a monitored recharge resistance value and/or a monitored recharge current of the energy storage device, to manage a recharge time of the energy storage device. The method may further include reducing the recharge time relative to a discharge time of the energy storage device, or simply minimizing the recharge time to achieve a certain recuperation of energy into the energy storage device. In one embodiment, a method is provided. The method includes affecting a change, over time, in a charging resistance of at least one energy storage device (e.g., a battery power source) of a hybrid power system comprising the energy storage device and at least one engine. The method further includes determining how a fuel 3 burn rate of the at least one engine is affected by the recharge resistance change in the energy storage device, and mapping fuel burn rate of the at least one engine to a plurality of partial states-of-charge (PSOC) windows of the at least one energy storage device based on the determining. The method may further include identifying a partial state-of-charge (PSOC) window of the plurality of partial states-of-charge windows of the at least one energy storage device, based on the mapping, that reduces the fuel burn rate of the at least one engine, and operating the energy storage device over the identified PSOC window. In one embodiment, a method is provided. The method includes estimating an effect of changing a recharge resistance of an energy storage device ^ P (e.g., a battery power source) on a fuel burn rate of an engine of a system comprising the energy storage device and the engine using a model of the system. The method further includes mapping the fuel burn rate to windows of partial states-of-charge of the energy storage device based on the estimating. The method may further include identifying a particular partial state-of charge (PSOC) window of the energy storage device, based on the mapping, that provides a minimal fuel burn rate of the engine as I a function of electrical power output of a generator coupled to the engine, and operating the energy storage device over the identified PSOC window. In one embodiment, a method is provided. The method includes determining a charge window of operation of an energy storage device (e.g., a battery power source), based at least in part on a profile of recharge resistance value vs. charge state of the energy storage device, and controlling charging of the energy gmk storage device based on the charge window of operation. In one embodiment, a method is provided. The method includes recharging an energy storage device (e.g., a battery power source) by applying a first recharge potential to the energy storage device when a recharge resistance value of the energy storage device is below a resistance threshold value (or, equivalently, when a recharge current value is above a current threshold value). The method further includes continuing to recharge the energy storage device by applying a second recharge potential to the energy storage device that is lower than the first recharge potential when the recharge resistance value of the energy storage device is above the resistance threshold value (or, equivalently, when the recharge current value is below 4 I the current threshold value). The method may instead include discharging the energy j storage device when the recharge resistance value of the energy storage device is | above the resistance threshold value (or, equivalently, when the recharge current value is below the current threshold value). In one embodiment, a system is provided. The system includes an energy ; storage device (e.g., a battery power source) configured to store DC electrical power [ and provide DC electrical power to a DC load. The system further includes a | regulator operatively connected to the energy storage device and configured to convert AC electrical power, from an AC electrical power source, to DC electrical power and provide the DC electrical power to the energy storage device and/or to the ^pr DC load. The AC electrical power source may include an electrical generator driven by a rotating mechanism. For example, the AC electrical power source may include an engine and generator set that is configured to generate AC electrical power. Other types of AC electrical power sources may be possible as well, in accordance with j various other embodiments. The system also includes a controller in communication with the energy storage device and the regulator. The controller may be operable to store a determined profile and/or map of recharge resistance value vs. charge state of the energy storage device, and determine a charge window of operation of the energy J storage device, based on the profile and/or the map, which conserves fuel used by the ; AC electrical power source. The controller may be further operable to cyclically turn the AC electrical power source on and off based on the charge window of operation. j The controller may be further operable to determine the profile and/or map by g g monitoring potential and recharge current of the energy storage device during j operation of the system. The controller may be further operable to direct the regulator to apply a first recharge potential to the energy storage device when a determined recharge resistance value of the energy storage device is below a resistance threshold value (or, equivalently, when a recharge current value is above a current threshold value), and direct the regulator to apply a second recharge potential to the energy storage device, which is lower than the first recharge potential, when a determined recharge resistance value of the energy storage device is above the resistance threshold value (or, equivalently, when the recharge current value is below the current threshold value). The controller may be further operable to determine a recharge 5 resistance value of the energy storage device by monitoring potential and recharge l current of the energy storage device during operation of the system. In one embodiment, a system is provided. The system includes an energy i storage device (e.g., a battery power source) configured to store DC electrical power and provide DC electrical power to a DC load. The system further includes a regulator operatively connected to the energy storage device and configured to regulate DC electrical power, from a DC electrical power source, and provide the DC ; electrical power to the energy storage device and/or to the DC load. The DC ) electrical power source may be a solar panel system or fuel cell energy system, for example. Other types of DC electrical power sources may be possible as well, in ^ P accordance with various other embodiments. The system also includes a controller in communication with the energy storage device and the regulator. The controller is operable to store a determined profile and/or map of recharge resistance value vs. charge state of the energy storage device, and determine a charge window of operation of the energy storage device, based on the profile and/or map, which conserves energy produced and/or stored by the DC electrical power source. The controller may be further operable to cyclically turn the DC electrical power source on and off based on the charge window of operation. The controller may be further operable to determine the profile or map by monitoring potential and recharge current I of the energy storage device during operation of the system. The controller may be j further operable to direct the regulator to apply a first recharge potential to the energy storage device when a determined recharge resistance value of the energy storage jk device is below a resistance threshold value (or equivalently, when a recharge current value is above a current threshold value), and direct the regulator to apply a second recharge potential to the energy storage device, which is lower than the first recharge potential, when a determined recharge resistance value of the energy storage device is above the resistance threshold value (or equivalently, when the recharge current value j is below the current threshold value). The controller may be further operable to determine a recharge resistance value of the energy storage device by monitoring potential and recharge current of the energy storage device during operation of the system. 6 ! j I I t BRIEF DESCRIPTION OF THE DRAWINGS Reference is made to the accompanying drawings in which particular § embodiments of the invention are illustrated as described in more detail in the description below, in which: FIG. 1 is an illustration of a first embodiment of a hybrid generator-battery power system for a telecommunications application (e.g., a base transceiver station); j FIG. 2 is an illustration of a simplified block diagram of a portion of the j hybrid generator-battery system of FIG. 1 that is configured to conserve fuel used by ,' the engine-generator; FIG. 3 provides a graphical illustration of the basic operation of a hybrid ^p* generator-battery power system such as the system shown in FIG. 2; I ** FIG. 4 provides a graph illustrating how the recharge resistance of the battery power source changes as a function of charge returned to the battery power source and age of the battery power source; FIG. 5 provides a graph illustrating an example embodiment of a cyclical charging/discharging methodology for the hybrid generator-battery power system of FIG. 2 based on the recharge resistance characteristics of FIG. 4; FIG. 6 illustrates two flow charts of two example embodiments of methods of adapting a partial state-of-charge (PSOC) window in a hybrid generator-battery i power system to achieve generator fuel savings; FIG. 7 provides a graph illustrating a comparison of load fraction of a generator of a hybrid generator-battery power system versus years of service; • FIG. 8 provides a graph illustrating an example embodiment of how a recharge potential (voltage) applied to a battery power source of a hybrid generatorbattery power system is controlled with respect to recharge current and age of the j battery; i FIG. 9 is an illustration of a second embodiment of a hybrid generatorbattery power system for a telecommunications application showing a first control 5 architecture in which the fuel saving methods described herein can be implemented; f FIG. 10 is an illustration of a third embodiment of a hybrid generator- j battery power system for a telecommunications application showing a second control { 1 I I I f architecture in which the fuel saving methods described herein can be implemented; 1 and | FIG. 11 is an illustration of a fourth embodiment of a hybrid generatorbattery power system for a telecommunications application showing a third control architecture in which the fuel saving methods described herein can be implemented. j DETAILED DESCRIPTION Embodiments of the present invention relate to improved fuel or energy 1 savings in hybrid energy storage power systems and enable an energy storage device } (e.g., a battery) in cyclic operation to be more effective at displacing run time of a ^ P primary power source (e.g., an engine-generator set) and reducing fuel or energy costs. The embodiments accomplish this by reducing recharge time of the energy storage device (ESD) through the application of a controlled potential, and/or primary power source loading, and/or battery recharge time management over the life of the system through manipulation of the charge state window of operation of the ESD. In general, fuel or energy savings of a primary power source can be increased by controlling the partial state-of-charge (PSOC) window of the ESD (e.g., a sodium metal halide type of battery) based on a recharge resistance profile of the ESD. Furthermore, fuel or energy savings can be increased by controlling a charging potential (voltage) applied to the ESD based on the observed recharge current of the ESD. Communication of either the key ESD behavioral information between an ESD unit and a primary power source (PPS) controller, or the suggested turn-on and turn- 0 t off events from the ESD unit to the PPS controller, using an adaptive control strategy, provides for optimization of the operating regime of the ESD to maximize fuel or energy savings, regardless of state of health of the ESD unit. Embodiments of the present invention provide a means to operate an ESD that optimizes between recharge time (providing maximum fuel or energy savings) and life. The operation in this case is envisioned, for example, as a high potential recharge with no limit on recharge current when the ESD is in a low resistance state and the ESD is of sufficient health. Once the ESD reaches a predetermined resistance value or has aged to a different state of health, a signal from the ESD unit (e.g., a battery management system) is issued to initiate the discharge or lower the recharge i 8 f I I I f potential if the system is not ready for ESD discharge. Other embodiments of the invention relate to a method to communicate or integrate with a master controller (e.g., a rectifier or a power interface unit, PIU) to initiate, terminate, or alter PPS j operation or recharging based on the state of health of the ESD. This communication i and/or integration covers a predefined algorithm loaded into the master controller, or { a digital signal communicated between the ESD management system and master controller, or a direct integral communication via a communication bus (e.g., CAN or Modbus). ; With reference to the drawings, like reference numerals designate identical or corresponding parts throughout the several views. However, the inclusion of like ^ P elements in different views does not mean a given embodiment necessarily includes • such elements or that all embodiments of the invention include such elements. The terms "battery", "battery power source", and "energy storage element" are used interchangeably herein, are all energy storage devices, and may or may not include t some form of a battery management system (BMS), in accordance with various embodiments. FIG. 1 is an illustration of a first embodiment of a hybrid generator-battery power system 100 for a telecommunications application (e.g., a base transceiver station). This embodiment shows four possible sources of power including an AC electrical grid 110, an engine-generator power source or engine-generator set (EGS) 120, alternate energy sources (e.g., solar, wind) 130, and a battery power source 140 which is an energy storage device (ESD). A transfer switch 115 allows transfer of jpH operation between the AC grid power source 110 and the EGS 120, as well as other alternative energy sources of AC electrical power that may be available. The EGS ? 120 runs on fuel (e.g., diesel fuel) provided by a fuel source 125 (e.g., a storage tank). The EGS is an AC electrical power source. Other types of AC electrical power { sources are possible as well in accordance with various other embodiments such as, for example, a wind energy system. Embodiments of the present invention are ; configured to operate the hybrid generator-battery system 100 to minimize fuel i consumption (or at least reduce fuel consumption relative to other possible modes of operation) to provide fuel savings to an operator of the system 100. An availability f 9 J j switch 135 allows for alternate energy sources 130, if available, to be switched in to a i DC bus 145 or an AC bus 155 of the system 100. J The system 100 also includes a power interface unit (PIU) 150 that > distributes AC power from the AC grid 110 or the EGS 120 to an AC bus 155. The AC bus 155 can provide AC power to drive AC loads 160 of the system such as, for example, lighting and air conditioning of a telecom base transceiver station (BTS). Furthermore, the AC bus 155 can provide AC power to a rectifier and/or a voltage regulator 170 which converts AC power to DC power and provides the DC power to the DC bus 145 to drive DC loads 180 of the system such as the radios, switches, and amplifiers of the telecom base transceiver station (BTS). ^ # The DC bus 145 also provides DC power from the rectifier 170 to charge the battery power source 140 and provides DC power from the battery power source 140 to the DC loads 180 as the battery power source 140 discharges. The controller 190 monitor various conditions of the system 100 and communicates with the EGS 120 to turn the engine of the EGS 120 on and off in accordance with a control logic of the controller 190. In accordance with various embodiments, the controller 190 may be a separate unit, may be a part of the PIU 150, or may be a part of a battery management system (BMS) of the battery power source 140. In accordance with other embodiments, the rectifier or regulator 170 may regulate DC power from a DC electrical power source (e.g., a solar energy system or a fuel cell energy system) instead of an AC electrical power source. The terms "rectifier" and "regulator" are used broadly herein to mean a device that conditions ^ 1 energy from a primary power source to provide DC electrical power to DC loads (e.g., DC loads 180) and to an ESD (e.g., the batteries 140). When the primary power source uses fuel, such as in the case of a diesel engine, a fuel savings may be achieved by employing the methods and techniques described herein. When the primary power source produces and/or stores energy such as, for example, a solar panel system, an energy savings may be achieved by employing the methods and techniques described herein. In general, a primary power source may provide AC or DC electrical power that is used by an ESD (e.g., by a DC battery power source) of the system. FIG. 2 is an illustration of a simplified block diagram of a portion of the hybrid generator-battery system 100 of FIG. 1 that is configured to conserve fuel used 10 by the engine-generator 120. FIG. 2 shows various system elements for providing DC power to a DC load 180 using only the EGS 120 and the battery power source 140 by cycling the engine of the EGS 120 on and off in such a way so as to conserve fuel. "Conserving fuel" can mean reducing or minimizing a fuel burn rate (e.g., using less fuel or energy over a defined period of time) and/or using less fuel or energy per unit of AC electrical power generated, for example. Other meanings of "conserving fuel" may apply as well, in accordance with various embodiments of the present invention. The controller 190 provides the control logic for operation of the system. j The controller 190 may be, for example, a logic controller implemented purely in J hardware, a firmware-programmable digital signal processor, or a programmable •fp- processor-based software-controlled computer. Again, the controller 190 may be a standalone unit, part of a power interface unit (PIU), part of a battery management I system (BMS), or part of some other portion of an embodiment of the system. ; During cyclical operation, when the EGS 120 is on, the EGS provides power to the DC load 180 and to the battery power source 140 during a charging part i of the cycle. When the EGS 120 is off, the battery power source 140 provides power ( to the DC load 180 during a discharging part of the cycle. The state of the battery power source 140 is estimated by observations of the potential and current of the battery power source 140. Specifically the series or recharge resistance profile is I learned or otherwise determined as a function of charge status. This characteristic is j then monitored and updated as the battery power source 140 ages. The control methodology can reside in any controller 190 that has access to the current and £ ^ voltage information of the battery power source 140, as well as access to the engine start/stop control signals. FIG. 3 provides a graphical illustration of the basic operation of a hybrid • generator-battery power system such as the system shown in FIG. 2. The graph 310 shows power output from or input to the battery power source 140 over time, where positive power indicates battery discharge and negative power indicates battery recharge. The graph 320 shows state-of-charge (SOC) for a particular implementation of the battery power source 140 over time. The graph 330 shows the relatively steady j operating voltage (e.g., -48 VDC) of the battery power source 140 over time. 1 1 j f When the graph 310 is maximum and flat (e.g., at level 315), the battery power source 140 is discharging its stored energy to the DC load 180 over the DC bus 145, and the engine of the EGS 120 is off. During this time, the state of charge (SOC) of the battery power source 140, as shown by graph 320, decreases as the battery discharges into the DC load 180. At a certain point 316, after the battery power ? source 140 has discharged a certain amount, the engine of the EGS 120 is turned on by the controller 190. While the EGS 120 is on, the EGS 120 is both recharging the battery power source 140 and is providing power to the DC load 180 over the DC bus 145 via the regulator (rectifier) 170. During this time, the SOC of the battery power source 140 increases, as shown in graph 320 and the battery power is in the negative region of the graph 310, indicating that power is flowing into the battery power source 140. Once the battery power source 140 recharges to a certain state at point 317, the EGS 120 is turned off again and the process repeats, forming a cyclic process. By i optimizing the cyclic process, fuel savings can be increased (i.e., a fuel burn rate of the EGS 120 can be decreased). | FIG. 4 provides a graph illustrating how the recharge resistance of the battery power source 140 changes as a function of charge returned to the battery t power source 140 and age of the battery power source 140. In general, when a battery S is in a low state of charge, the recharge resistance is low, allowing the battery to build ; up charge relatively quickly for a given applied charging voltage (potential). I However, as the charge returned builds up in the battery, the recharge resistance of the J g battery increases and the rate of charging slows down, for the given applied charging voltage. Furthermore, as a battery ages, the entire curve of recharge resistance vs. charge returned tends to shift upward. As a result, the recharge resistance ends up i affecting the amount of time the engine of the EGS 120 has to be on to re-charge the j battery power source 140 and, the longer the EGS 120 is on, the more fuel that is I burned. I In particular, for sodium metal halide type batteries, the curve representing recharge resistance vs. charge returned can be quite dynamic due to the nature of I sodium metal halide type batteries. In accordance with various embodiments, the metal in a sodium metal halide type of battery may be one or more of iron, nickel, 12 I zinc, and copper. The halide may be chloride, for example. In general, sodium metal halide type of batteries provide a first-in/first-out type of operation. For example, as a sodium ion is passed into the cathode mix, the sodium ion finds the first site it can ; possibly bind to and proceeds to bind. As a result, recharging resistance of a sodium metal halide type of battery tends to increase as state-of-charge increases. The recharge resistance vs. charge returned to the battery power source 140 is of keen interest to the system, as this highly effects time on recharge. Observing the recharge resistance profile will inform the engine start/stop controller 190 of the best or most desirable battery charge window. In this application, it is f often the case that the battery power source 140 is operated over a small region of its 9 * tota' charge window. This is called partial state of charge (PSOC) operation. The EGS 120 is controlled on and off by way of certain thresholds of acceptable battery power source operating ranges. If the recharge resistance characteristic suggests that a smaller charge state window is warranted, the EGS 120 will be controlled to keep the battery power source 140 operating within the proper PSOC band. Using sodium metal halide type batteries, recharge-to-resistance profiles or functions can be used to maintain fast recharge and provide a recharge time-to-discharge time ratio that maintains fuel savings. FIG. 5 provides a graph illustrating an example embodiment of a cyclical charging/discharging methodology for the hybrid generator-battery power system of FIG. 2 based on the recharge resistance characteristics of FIG. 4. The battery power source 140 is operated over the PSOC window 510 as shown in FIG. 5. For example, 0k at setpoint 511 the battery power source 140 has discharged to a lower state-of-charge (SOC) level 513 after supplying power to the DC load 180. The state-of-charge (SOC) of the battery power source is determined by the controller 190 based on the current feedback from the battery power source 140 to the controller 190. In general, the SOC can be estimated by the controller 190 by determining the current going into : and out of the battery power source 140. This may be done by implementing a charge counter functionality in the controller 190 that effectively counts charge in units of, for example, amp-hours. The setpoint 511 defines the lower limit of the PSOC window 510 and is an indicator to the controller 190 to start the EGS 120. When the EGS 120 starts at 13 I setpoint 511, power is supplied to both the DC load 180 and to the battery power source 140 to charge the battery power source 140. The battery power source 140 is charged over a time Tc until the upper setpoint 512 is reached at an upper SOC level 514, where the EGS is stopped by the controller 190. At setpoint 512, the DC load 180 is driven by the battery power source 140 as the battery power source discharges back to the lower setpoint 511 over a time T